Conduit bending multipliers

The offset multiplier is how many times the rise you space your marks: rise × multiplier is the distance between the two bends. It depends only on the angle. Pick an angle above for the exact figure, or read the table below for the five common angles beside the rounded rules the trade carries.

The reference table

Exact multiplier and shrink per inch of rise on the left, the shop rule of thumb on the right. The exact values come straight from the trigonometry, so they are correct to as many places as you care to read.

Worked example

You need a 5 inch offset at 22.5°. The exact multiplier is 2.613, so the marks go 5 × 2.613 = 13.06 inches apart. The field rule of 2.6 would give 13.0 — a sixteenth short, which nobody would notice here. On a 20 inch offset the same rounding is a quarter inch, which you would. That is the whole argument for keeping the exact figure to hand.

The formula

The multiplier is one over the sine of the angle; the shrink per inch is that less one over the tangent:

multiplier = 1 ÷ sin θ · shrink/in = 1 ÷ sin θ − 1 ÷ tan θ

The offset is the rise of a right triangle and the pipe between the marks is the hypotenuse, so the hypotenuse is rise ÷ sin θ — that is the multiplier. The horizontal run the pipe covers is rise ÷ tan θ, and the difference between the hypotenuse and that run is what the pipe pulls back: the shrink.

Questions

What is the multiplier for a 30° offset?

Exactly 2. The multiplier is 1 ÷ sin(angle), and sin 30° is 0.5, so 1 ÷ 0.5 = 2. Multiply your rise by 2 to get the distance between the two marks. Most benders learn 30° first for exactly this reason — the mental math is trivial.

Why are there two numbers for each angle?

The first is the exact multiplier from the trigonometry; the second is the rounded figure the trade carries in its head. The field rule for 22.5° is 2.6, the exact value is 2.613 — close enough for a small offset, off by enough to matter on a long one.

Does the multiplier depend on the conduit size?

No. The multiplier comes only from the angle, so 30° is ×2 whether you are bending half-inch EMT or two-inch rigid. Take-up and gain change with the bender and the pipe; the offset multiplier and the shrink per inch do not.

When does the rounding start to matter?

On large offsets and tight runs. An eighth of an inch of drift per inch of rise is invisible on a 3 inch offset and obvious on a 20 inch one. Use the exact figure when the offset is big or the run is short enough that the far end has no slack.

Related tools

• Conduit offset calculator — put a rise in and get the marks.
• Three-point saddle calculator — uses the outer-angle multiplier.
• Rolled offset calculator — the multiplier on a tilted jog.